1. Field of the Invention
The present invention relates to the field of shock testing and more particularly to an apparatus and method for subjecting a test specimen to a high-g shock in the laboratory to simulate the conditions the specimen might encounter in actual use.
2. Description of the Prior Art
Of course, a test specimen may be tested at a Proving Ground under substantially identical conditions as will be encountered in actual use. However, the cost of transporting the specimen to the Proving Ground is very high and only one or two tests per day can be performed. Accordingly, it is desired that a laboratory test be provided so as to minimize expense and increase convenience, so that several tests per day can be performed.
Laboratory shock testing apparatus, utilizing a device identified as a Hopkinson bar is known in the art. Such apparatus is described in a paper entitled xe2x80x9cThe Use of a Beryllium Hopkinson Bar to Characterize a Piezoresistive Accelerometer in Shock Environmentsxe2x80x9d presented by Vesta I. Bateman, Fred A. Brown, and Neil T. Davis of Sandia National Laboratories in Albuquerque, N. Mex., in the 1996 Proceedings of the Institute of Environmental Sciences on pages 336-343. This prior system employs a Hopkinson bar, i.e., a perfectly elastic homogeneous bar of constant cross-section which has first and second end surfaces substantially perpendicular to the length. A test specimen, in this case an accelerometer, is mounted on the surface at the first end and the bar is then impacted on the surface at the second end by a projectile that is fired by an air gun down a long tube to produce a shock wave that travels the length of the bar and applies a high-g force to the specimen. The prior art has several disadvantages, among which is the fact that to produce a sufficiently high-g force, a relatively large projectile traveling at high speed must be used (the higher the force desired, the greater the size and/or speed of the projectile). This requires a significantly long tube (e.g., up to 40 feet) for the projectile to reach the desired speed necessary to produce a high-g force (for example, say above 10,000). Thus, a great deal of laboratory space needs to be provided, which is costly and inconvenient. Furthermore, the greater size and speed of the projectile introduces greater danger in performing the test. Also, the period or duration of the high-g shock varies inversely with the amplitude of the shock (i.e., the higher the g-force desired, the shorter the duration of the shock; usually considerably less than one second).
The present invention uses step relaxation of a very stiff spring to attain the high-g levels. The spring, a fixed beam which, in a preferred embodiment comprises an I-beam of high strength aluminum, is used to mount the test specimen. The beam is put under high strain, such as by applying a large force, tending to bend the beam to near capacity (i.e., its yield point), and then the stored energy in the beam is released by suddenly removing the force to produce a high-g shock that has a significantly long duration (for example about one second) which is independent of the magnitude of the shock. Furthermore, in the present invention it is simple, inexpensive, and does not require excessive laboratory space.